1. Field of the Invention
The invention relates to a sliding bearing using a liquid lubricant and a rotary anode X-ray tube using the sliding bearing.
2. Description of the Related Art
A rotary anode X-ray tube used in an imaging diagnostic system and the like is used at a high temperature and in a vacuum, and moreover the anode target is rotated at high speed. Such a rotary anode X-ray tube is structured as disclosed in Japanese Patent 2960089 such that the rotation axis that supports the rotary anode is supported by a sliding bearing which uses a liquid metal as a lubricant. In order to further reduce the X-ray tube in size and weight, it is required to cool the rotation target through a liquid metal. For this reason, a proposal has been made for a structure such that a gap is set between the back of the rotation target and a fixed shaft and a liquid metal is injected into the gap as a heat transfer fluid to thereby cool the rotation target.
With the rotary anode X-ray tube, when the X-ray tube apparatus is operated, the anode target reaches a high temperature due to entry of heat to it. That is, the anode target is irradiated with an electron beam and consequently reaches a high temperature. In particular, the electron bombardment surface (focal point) which is struck by electrons reaches a high temperature. For this reason, the anode target must be maintained at temperatures below the melting point of its material.
From such a point of view, techniques to cool the anode target have been developed. Among the techniques is one which uses a liquid metal as a heat transfer fluid in the vicinity of the electron bombardment surface and transfers the heat of the anode target to cooling water within a cooling box, thereby cooling the anode target.
However, the conventional rotary anode X-ray tube using the liquid metal as a heat transfer fluid for cooling has the following problem:
With the cooling mechanism which uses the liquid metal as a heat transfer fluid for cooling, it is required to surely introduce the liquid metal used as a lubricant into the gap between the cooling box integral with the fixed shaft and the anode target. The amount of the liquid metal to be filled in is limited so as not to cause leakage from the seal portion when the rotating body is stopped. When the rotating body starts rotating, the liquid lubricant is pressed against the inner part of the rotating body due to centrifugal force and then introduced from the fixed shaft into the gap between the cooling box and the anode target.
However, the liquid metal needs to pass through the narrow gap in the dynamic pressure type bearings; therefore, it takes long to introduce the liquid metal into the gap between the cooling box and the anode target.